The objective of this research is to obtain a better understanding of the factors which are important in lung development and differentiation. Fetal rat lung placed in organ culture provides the principal method for approaching the problem. Selected in vivo experiments will be conducted to confirm the in vitro findings. The experiments proposed will study the biochemical pathways involved in the synthesis, storage, and secretion of disaturated phosphatidylcholine, the major lipid component of pulmonary surfactant. Rat fetal lung removed at 15.5 days gestation appears to be pre-programmed for abruptly turning on disaturated phosphatidylcholine synthesis after 6 days in culture. Using inhibitors of RNA, protein and glycoprotein synthesis, the nature of this pre-program will be evaluated and specific mechanisms involved in the abrupt increase in disaturated phosphatidylcholine accumulation will be studied to provide a better understanding of the biochemical differentiation of this pathway. The research plan includes studying the fetal lung by light and electron microscopy to be able to make structure-function correlations at critical points during differentiation.